Solar cell module

ABSTRACT

The solar cell module comprises a housing (10) having at least one aperture (15) with associated concentrator (12). Light energy, propagated along the optical principal axis (45) of the concentrator, passes through the aperture (15) and is concentrated on a primary photoactive area (20). The solar cell module moreover comprises a secondary photoactive area (30) arranged in the housing (10) so as to be illuminated by light energy which is propagated in a direction different from the optical principal axis (45) of the concentrator.

FIELD OF THE INVENTION

The invention concerns a solar cell module comprising a housing with atleast one aperture with an associated concentrator, where light energy,which is propagated along the optical principle axis of theconcentrator, passes through the aperture and is concentrated on aphotoactive area.

BACKGROUND OF THE INVENTION

In recent years much research work has been devoted to solar cells, andthis has i.a. entailed an increased interest in solar cell modules ofthe above-mentioned type. These modules have a significant advantage,since the active area of the module is determined by the area of theconcentrator, and such a concentrator may e.g. be a lens or a mirror.The concentrator is expediently a Fresnel lens, since these can be madeof a plastics material, thereby minimizing the manufacturing costs. Atthe same time, Fresnel lenses have a significant advantage, since thelens thickness is reduced, so that the concentrator is very light inthis case. When the optical principal axis of the concentrator isdirected to the sun, the latter will be imaged in the focal point of theconcentrator. Light energy having a very high intensity level will beconcentrated around the focal point, so that the solar cell, which is toconvert this light energy into electric energy, must be of a very highquality, e.g. made of monocrystalline silicon or gallium arsenide. Forbest possible utilization of the area of this solar cell, a secondoptical element is usually positioned in front of the solar cell, saidoptical element being constructed such that the light energy isdistributed evenly over the active areas of the entire solar cell. Aplurality of solar cell modules of this type are then assembled toprovide a solar cell panel of matrix shape. Since, owing to the imagingproperties of the concentrator, the solar cells only receive lightenergy which is propagated along the optical principal axis of theconcentrator, the panel must be mounted on a device so that the panelcan follow the travel of the sun across the sky. This technique is knownand described in e.g. "Conference Record of The Twentieth IEEEPhotovoltaic Specialists Conference, 1988, Volume II", pages 1138-1143by Don Carroll.

Solar cell panels of this type operate satisfactorily when the opticalprincipal axis of the concentrator is directed to the sun. In cloudyweather the visual field of the panel does not contain a powerful lightsource in the form of the sun, since the incident light will be in theform of diffuse light from all directions. The advantages of theconcentrator by concentrating light from an area corresponding to e.g.500 times the area of the solar cell, are thus gone. A solar cell panelof that type is therefore useful only in regions where a reasonablenumber of sunshine hours per day is certain with some statisticalprobability, it being thus certain that the solar influx is mainlydirect influx. Therefore, it is not attractive to use such a solar cellpanel in large parts of the world, since it must be consideredunreliable depending upon the light conditions.

OBJECT OF THE INVENTION

The object of the invention is to improve solar cell panels of theabove-mentioned type such that they also give a reasonable electricoutput even if they are not irradiated directly by the sun.

SUMMARY OF THE INVENTION

This object is achieved in that each individual solar cell module isprovided with a secondary photoactive area which is arranged in thehousing so as to be irradiated with light energy, which is propagated ina direction different from the optical principal axis of theconcentrator.

The primary photoactive area is expediently provided on the rear wall ofthe housing on the optical principal axis of the concentrator, while thesecondary photoactive areas can then expediently be provided at least onpart of the other inner walls of the housing. Light passing theconcentrator in a direction different from its principal axis can thenbe collected and converted to electric energy by solar cells provided onthe walls of the housing. The parts of the walls of the housing whichare not coated with solar cells, may then be provided with reflectinglayers, whereby the light energy is reflected until it is collected by asolar cell. If the solar cell module is incorporated in a solar cellpanel together with several other modules, the side walls facing theother modules may be transparent so that light can freely move from onesolar cell module to another, thus enabling a reduction in the area ofsolar cells in the secondary photoactive areas.

The concentrator may expediently be provided in the inlet opening of thehousing and be a Fresnel lens. Other imaging components may be used as aconcentrator, including ordinary lenses, holographic lenses, and concavemirrors. The primary photoactive area preferably comprises a highquality solar cell, e.g. made of monocrystalline silicon or galliumarsenide. To increase the efficiency of this solar cell, it may beconnected to a cooling element, a Peltier element. The secondaryphotoactive area is not illuminated by light of the same high intensityas the primary photoactive area, so that the solar cells used here maybe made of cheaper materials, e.g. polycrystalline or amorphous silicon.

The housing may expediently be constructed so that its walls conform tothe imaging of the sun by the concentrator on the primary photoactivearea. Thus, the housing has a cross-section which decreases along theoptical principal axis of the concentrator toward the rear wall of thehousing. The housing thus has the shape of a truncated cone or pyramidwith the concentrator arranged in the large base face of the housing andthe primary photoactive area in the small base face.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained more fully below in connection withpreferred embodiments and with reference to the drawings, in which

FIG. 1 schematically shows an embodiment of a solar cell moduleaccording to the invention,

FIG. 2 schematically shows a second embodiment of a solar cell moduleaccording to the invention,

FIG. 3 shows a solar cell panel with a plurality of solar cell modulesaccording to the invention,

FIG. 4 shows a preferred embodiment of a primary solar cell for use inthe solar cell modules shown in FIGS. 1 and 2, and

FIG. 5 shows an electrical equivalent diagram of a preferred embodimentof a solar cell module according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a preferred embodiment of a solar cell module according tothe invention, the solar cell module having a housing 10. The walls ofthe housing 10 may e.g. be made of plastics. At its one end the housing10 has an aperture 15 in which a concentrator 12 is positioned. Theconcentrator 12 is here an optically imaging element focusing incidentlight on a primary solar cell 20. The concentrator 12 has an opticalprincipal axis 45, a light beam 40, which propagates in parallel withthis optical principal axis 45, passing through the concentrator 12 andbeing imaged on the primary solar cell 20. The area of the concentrator12 thus decides the size of the light beam 40 and thus the quantity oflight passing into the housing 10. It is known to move such a solar cellmodule so that the optical principal axis 45 of the concentrator 12 ismade to aim at the sun. Part of the other inner walls of the solar cellmodule is here coated with a secondary photoactive area 30 consisting ofdensely positioned solar cells 32, which are shown to be circular. Theremaining part of the walls of the housing 10 may then expediently becoated with a reflecting material 35, which may optionally also beapplied to the walls rearwardly of the positioned circular solar cells32. The solar cell module can hereby collect energy from diffused lightpassing the concentrator in a direction different form its opticalprincipal axis. This diffuse light may be due to refraction of the solarlight in the atmosphere before the solar light reaches the solar cellmodule. This refraction usually takes place in water vapor, such as bypassage through cloud layers, as well as in foggy weather. The diffuselight may also comprise light reflected from the surface of the earth.

This configuration ensures a higher solar cell module efficiency,efficiency being here understood to mean the relationship between theelectric power that the solar cell module can generate and the power ofthe light energy passing the concentrator 12. Here, the primaryphotoactive area 20 consists of a high-effective solar cell ofmonocrystalline silicon or gallium arsenide. Such a solar cell isrelatively expensive, but is necessary here since the light intensitypower is relatively great, which gives a relatively high operatingtemperature. This operating temperature may be reduced considerably bycooling of the solar cell 20 which may e.g. take place by using aPeltier element, which, as shown in FIG. 4, may be incorporated as anintegrated part of the solar cell. It is hereby possible to lower theoperating temperature of the solar cell to a temperature at which theefficiency of the primary solar cell is relatively high.

In this embodiment the secondary photoactive area 30 is composed of aplurality of circular solar cells 32, some of which are shown. Thesesolar cells 32 may expediently be connected in series; these connectionsare omitted for clarity and must be considered prior art since they areknown from ordinary solar cell panels. If the secondary photoactive areacontains a large number of individual solar cells, these may beassembled to parallel-connected chains of series-connected solar cells.Each of these chains is then connected to an electric regulator, whichwill be mentioned in connection with FIG. 5. Since the individual solarcells are connected in series, it is expedient that the individual solarcells comprise a parallel-connected bypass diode which is integrated inthe actual solar cell. In case of failure the solar cell will thus notinterrupt the series connection, but instead operate as a diode. Thesolar cells 32 are illuminated by light having a low intensity withrespect to the light illuminating the primary solar cell 20. Therefore,it is not necessary that the solar cells 32 are of the same high qualityas the solar cell 20, but may be made of a considerably cheapermaterial. The solar cells 32 may e.g. be made of amorphous silicon orpolycrystalline silicon.

FIG. 2 shows an alternative embodiment of a solar cell module accordingto the invention. The housing 10a is here given the shape of a truncatedpyramid or a truncated cone, it being adapted to the shape of theimaging of the plane beam 40 by the concentrator 12a. In this case theconcentrator 12a is a Fresnel lens, which is expedient to use owing toits low weight when the solar cell module is to be mounted on a deviceand be caused to follow the movements of the sun. The actual housing ishere cone-shaped, the primary solar cell 20 being arranged at the vertexof the housing. Part of the inner side of the housing 10a is providedwith a secondary photoactive area 32a, which, like in the previous case,may consist of circular solar cells or may consist of a contiguous solarcell made e.g. by sputtering. Here too, part of the inner wall of thehousing 10a is provided with a reflecting material 35a. A beam 41,propagating in a direction different from the optical principal axis ofthe concentrator 12a, is refracted in the concentrator 12a and continuesinto the housing 10a. The beam 41 continues into the housing 10a andimpinges on the inner wall and is reflected toward the secondaryphotoactive area 32a in a point on the reflecting face 35a.

FIG. 3 shows how the solar cell module shown in FIG. 1 may be used forconstructing a solar cell panel. In the shown example the panelcomprises a rear plate 80, with a contact area 90 for each solar cellmodule. Each of these solar cell modules comprises a housing with anopening mounting a concentrator 12 in the form of a Fresnel lens, theconcentrator 12 imaging incident light on a primary photoactive area 20.The solar cell panel is composed of solar cell modules arranged closelyspaced to one another, and the adjacent sides 38 of these modules mayexpediently be transparent so that the light is allowed to move freelyfrom one module to another. On the other hand, the outer sides 14 of thepanel are expediently opaque. The outer sides 14 are expediently fullyor partly coated with a secondary photoactive area on the inner side,which is also the case for the bottom 15. The rear side of each solarcell module may expediently be provided with complementary contact meansensuring both mechanical and electric contact to the contact areas 90 onthe base plate 80 of the solar cell panel. The secondary solar cell areamay be shaped as shown in FIG. 1. For clarity, two of the solar cellmodules have been removed from the solar cell panel.

It is possible to provide the solar cell module with transparent outersides 14, so that light is also permitted to pass that way. The innerside of the transparent sides 14 may then be coated with bi-facial solarcells, thereby making it possible to increase the power from such asolar cell module additionally.

FIG. 4 schematically shows a cross-section of a solar cell 20. Thissolar cell is constructed with a monocrystalline silicon crystal 210, onwhose upper side incident light is focused by the concentrator 12 shownin FIG. 1. A copper housing 220, also serving as an electric and thermalconductor, is soldered onto the contact areas on the front sides of thesolar cell 210. The copper housing 220 has electric contact to a contactleg 270. The rear side of the solar cell 220 is soldered to a copperheating bridge 230. This copper heating bridge, too, serves as a thermaland electric conductor. The copper heating bridge 230 has electriccontact to an electric contact leg 260. The rear side of the copperheating bridge 230 is soldered to a ceramic front side of a Peltierelement 240. The ceramic rear side of the Peltier element 240 is inthermal contact with a cooling element 250 having cooling fins. Thecooling element 250 is moreover in contact with the copper heatingbridge 230.

The Peltier element 240 has two +/- wires 245. The primary photoactivearea 20 thus has four output terminals--two from the Peltier element andtwo from the actual solar cell. As will be seen from the equivalentdiagram on FIG. 5, these may be run to an electric voltage regulator,which is also connected to the row of solar cells in the secondaryphotoactive area. The primary cell and the secondary cells apply voltagein response to the intensity of incident light. When subjected to acertain temperature difference between the front side and the rear side,the Peltier element per se generates a current, while serving as acooling element. If additional cooling of the primary solar cell isneeded, voltage may be applied to the Peltier element to improve itscooling properties. The electric regulator may be provided with meansfor controlling the voltage across the Peltier element, so that this isprovided with voltage in case of high incident light intensity, therebyoperating as an active cooling element, or operates passively andgenerates a current thereby in case of low incident light intensity.

The concentration factor, the ratio of concentrator area to primarysolar cell area, is typically between 20 and 500. With the presentinvention it is expedient that the factor is as great as possible.

It has been found possible by tests to convert more than 25% of thelight energy passing the concentrator into electric energy.

I claim:
 1. A solar cell module comprising a housing having at least oneaperture, a concentrator, and a primary photoactive area, theconcentrator being associated with the aperture so that light energypropagated along the optical principal axis of the concentrator passesthrough the aperture and is concentrated on the primary photoactivearea, and a secondary photoactive are arranged in the housing so as tobe irradiated by light energy which is propagated in a directiondifferent from the optical principal axis of the concentrator.
 2. Asolar cell module according to claim 1, wherein the concentrator ismounted in the aperture of the housing.
 3. A solar cell module accordingto claim 1 or 2, wherein the concentrator is a Fresnel lens.
 4. A solarcell module according to claim 1, wherein the housing has inner walls,including a rear wall, and the primary photoactive area is provided onthe rear wall of the housing on the optical principal axis of theconcentrator, and the secondary photoactive area is provided at least onpart of the other inner walls of the housing.
 5. A solar cell moduleaccording to claim 4, wherein at least a part of the walls of thehousing is transparent.
 6. A solar cell module according to claims 4,wherein at least a part of the walls of the housing is reflecting.
 7. Asolar cell module according to claim 4, wherein the housing has across-section decreasing transversely to the optical principal axis ofthe concentrator toward the rear wall of the housing.
 8. A solar cellmodule according to claim 7, wherein the housing has the shape of atruncated cone or pyramid.
 9. A solar cell module according to claims 1,wherein the primary photoactive area comprises a solar cell made ofmonocrystalline silicon or gallium arsenide.
 10. A solar cell moduleaccording to claim 9, wherein the solar cell in the primary photoactivearea is mounted on a Peltier element.